Homework Help: Angles between vectors

1. Aug 9, 2008

pokerfan91

ive got the formula down as angle=cos-1[(u.v)/(||U||||v||)

with my u as 3,1,2 and v as -2,3,4 this gives me cos-1[(-6i+3j+8k)/root406) where do i go from here?

2. Aug 9, 2008

Defennder

What is dot product supposed to give you? What did you get instead?

3. Aug 9, 2008

pokerfan91

that is what the dot product is ment to give me isnt it i just have no idea where to go after that to get the angle

4. Aug 9, 2008

nicksauce

No it isn't. The dot product is supposed to give you a scalar (ie a number), but you wrote that it gives you a vector. You can't very well take the arccosine of a vector can you?

5. Aug 9, 2008

HallsofIvy

$$a\vec{i}+ b\vec{j}+ c\vec{k}\cdot d\vec{i}+ e\vec{j}+ f\vec{k}$$
is defined as
$$ad+ be+ cf$$
a number, NOT as
$$ad\vec{i}+ be\vec{j}+ cf\vec{k}$$